Circuits which are of the type mentioned, excepting for the group filter, are described in an article by Ivan Koval and Georg Gara: "Digital MF Receiver Using Discrete Fourier Transform" (I.E.E.E. Transaction on Communications, Vol. COM-21, No. 12, December 1973, and is an article by Fritz G Braun: "Nonrecursive Digital Filters for Detecting Multi-Frequency Code Signals" (I.E.E.E. Transactions on Acoustics, Speech and Signal Processing", Vol. ASSP-23, No. 3, June 1973).
The selectivity requirement for multi-frequency code (MFC) receivers according to the CCITT specification for the signal system R2 is to a major extent determined by the requirement that operation for single tone may not occur. For these requirements to be fulfilled merely with the selective receivers, the time interval for calculating the correction must be about 30 ms. If an output decision is based on comparisons of the calculated correlations mutually as well as with some fixed threshold value, the risk is great that a short pulse of single tone or of a MFC signal causes operation if solely a 30 ms. interval is required for approval. Furthermore, the release time will be long if the same set of coefficients and thereby the same time interval is used for detecting release.
In the first of the above mentioned articles, there is proposed as a decision condition that two consecutive calculation intervals shall give identical results (same frequencies are present or there is an absence of signal) which accordingly makes the changeover to operation and release slower. As protection against interference it is proposed that the sum of the two greatest, selectively calculated energies be compared with the difference between the total energy and this sum.
A requirement according to the CCITT R2 specification is that simultaneous signalling in the send direction shall not affect the receiver. Frequencies which are used in signalling in the send direction must therefore be filtered away. If this is achieved with a group filter which is common to the selective receivers, the selectivity requirement for these may be reduced considerably, not only because the effect from signals in the send direction is eliminated at the input to the selective receivers, but also because the previously mentioned measure for protection against noise signals can be employed, since the mean value of the signal after the group filter can be calculated and a variable decision threshold obtained. This threshold is related to the total energy in the signal after the group filter, i.e. within the frequency band for the expected signals in the receive direction.
In an MFC receiver according to the CCITT R2 specification, each calculation interval for the correlations could be reduced to 15 ms. with the aforementioned measure for protection against noise signals, but the release time would still be close to 30 ms. even if only one calculation interval with the absence of signal were to be recognized as release. No indication of whether a short interruption can have disturbed the correlation calculation is obtained, which can result in that a single tone subjected to a short interruption is recognized as the presence of two adjacent MFC frequencies.